Constant current device



June 2, 1953 J. w. GRAY 2,640,962

CONSTANT CURRENT DEVICE Filed March 5, 1946 PLATE VOLTAGE SUPPLY UNREGULATED VOLTAGE ZI-i; 23 30 n INVENTOR.

JOHN w. GRAY ATTORNEY Patented June 2, 1953 CONSTANT CURRENT DEVICE John W. Gray, Cambridge; Mass., assignor, by mesne assignments, to the United States of America as Navy represented by the Secretary of the Application March 5, 1946,Serial No. 652,196

1 Claim. (Cl; 3234).

This invention relates to current stabilizers and more particularly to D. C. constant current devices.- I

In many applications such" as the field coils of tachometer-generators of the type used in many aircraft installations, it is essential that the field current be maintained very constant. As the temperature, and hence the resistance, of these field coils vary over a wide range in such an installation, and as the voltage for the field windings is generally obtained from an unregulated voltage supply, the current variations tend to be too large for the most satisfactory operation of the tachometer. It, therefore, becomes desirable to use a current stabilizing device to insure a constant current in the field coils and hence more satisfactory operation of the instrument.

This invention achieves the above required results with an economical use of parts and a small consumation of power. Another feature of this invention is that the current value at which stabilization occurs may be varied according to variations in a control voltage. The variations in the current may be made directly proportional to variations in this control voltage over a wide range of voltages. Although the application of this invention has been described with particular reference to its use with a tachometer-generator field, it is not to be considered as a limiting, but merely as an illustrative application.

A primary object of this invention is to generally improve constant current devices.

Another object of this invention is to provide a current stabilization device to maintain constant current through a resistance despite changes in that resistance and changes in the voltage of the current source.

A further object is to provide a constant current device suitable for use with direct currents.

A still further object is to provide a constant current device suitable for use in the field circuit of small generators.

A still further object is to provide a current stabilizing device in which the current may be varied according to variations in a control voltnected directly to the cathodes of triode electron tubes l5 and I6, and to ground through resistor l4. s The grid of triode l5 connects 'to ground through resistor20 and to the plate voltage supply at terminal 22 through resistor 2|. Theplate of triode l5 connects-through plate load resistor 23 to the plate voltage supply at terminal 22 and also to ground through the series combination of resistor 24 and resistor 25. The grid of triode l6 connects to the junction point of resistors 24 and 25, and also to the plate of triode I 5 through condenser 26. The plate of triode l6 connects through resistor 30 to the plate voltage supply at terminal 22 and also to ground through the series combination of resistor 3| and resistor 32. The grid of triode I3 connects to the junction of resistor 3| and resistor 32.

In the operation of this embodiment of the invention, it is desired to maintain a constant current flow through resistor I l despite changes in its resistance and changes in the voltage at terminal l2. This current is the plate current of triode l3 and also flows through the constant resistance of resistor l4, across which is developed a voltage proportional to the current to be maintained constant. A variation in the current through resistor ll causes a change in voltage across resistor [4 which is coupled directly into the cathode of triode l5. Triode l5 and I6, and related circuit elements, constitute a two stage D. C. voltage amplifier having positive voltage feedback due to common cathode resistor [4, resulting in a very large gain in the amplifier. The output of the amplifier is taken from the junction of resistors 3| and 32, and connected to the grid of triode l3. The phase of the signal output from the amplifier is such that triode l'3, responding to its grid voltage variations, tends to compensate for the current change that originally caused the error voltage across cathode resistor I4. Condenser 26 is included in the circuit to prevent the positive voltage feedback in the amplifier circuit from causing high frequency oscillations.

Changes in the plate voltage supply at terminal 22 results in variations in the grid to cathode voltage of triode l5 which appears on the grid of triode I3 in such a phase as to cause the plate current, and hence the stabilized current in resistor H, to follow in direct proportion to the changes in the plate supply voltage at terminal 22.

This invention need not be limited to the details shown in the foregoing specification which are considered to be illustrative of one embodiment thereof. The scope of the invention is defined by the appended claim.

What is claimed is:

A constant current device comprising a first resistance through which the current is to be maintained constant, a first electron tube having at least a catlmde, an anode, and a control grid, said first resistance constituting the plate load resistance of said first electron tube, a second resistance in the cathode circuit of said first electron tube across which is developed a variable voltage depending upon variations in the current in said first resistance, a second and third electron tubes each having at least a cathode, an anode, and a control grid, said second and third electron tubes comprising a two stage amplifier, the cathode to grid circuits of said second and third electron tubes including a common cathode resistance to provide positive voltage feedback, a voltage divider to provide a fixed voltage to the cathode-control grid circuit of said second electron tube, means to apply said variable voltage to the cathode-control grid circuit of said second electron tube, variations in said variable voltage when compared with said fixed voltage producing an error voltage in the cathode-con- References Cited in the file of this patent UNITED STATES PATENTS timber Name Date 2,120,884 Brown June 14, 1938 2,197,934 Koch Apr. 23, 1940 2,274,364 Gardiner Feb. 24, 1942 2,394,891 Bowie Feb. 12, 1946 2,447,507 Kenyon Aug. 24, 1948 2,484,724 Paradise et al. Oct. 11, 1949 OTHER REFERENCES Current Stabilizers, by Van Sooyoc let aL, published in Proceedings of the I. R. E. vol. 32 No. 7 July 1944, pages 415-418 inclusive, a copy may be found in Division 26 171-312-1729. 

